An X-ray CT apparatus (X-ray computed tomography imaging apparatus) irradiates an examinee, such as a patient, with X-rays and detects X-rays transmitted through the examinee. A data collecting device of the X-ray CT apparatus then collects X-ray transmission data which is based on the amount of X-rays detected. Thereafter, the X-ray CT apparatus performs reconstruction processing on the X-ray transmission data, and generates a slice image (a tomographic image) of the examinee.
An example of such an X-ray CT apparatus which has been developed is configured to image an examinee using an X-ray irradiator and an X-ray detector located opposite each other with the examinee on a table in between, while rotating them about the body axis of the examinee. This X-ray CT apparatus performs imaging by shuttle helical scan. In the shuttle helical scan, the examinee is imaged while a table on which the examinee lies down is moved with its moving direction being changed alternately between two directions along the body axis of the examinee i.e., a direction from the feet to the head and a direction from the head to the feet.
In this shuttle helical scan, a certain target region of the examinee on the table is imaged by collecting X-ray transmission data based on a time period determined by the number of views (a time period converted from the number of views) with the moving speed of the table being constant (within an allowable speed range). Note that the number of views is obtained from the number of rotations of the X-ray irradiator and the X-ray detector about the body axis of the examinee.
In the shuttle helical scan, the shuttling movement of the table tends to make the movement of the table inconstant. Thus, the distance travelled by the table within a certain time period (i.e., the moving speed of the table) tends to vary. Further, the rotational speed of the X-ray irradiator and the X-ray detector might also vary to change the width covered by one view. Thus, when imaging is controlled based on the number of views (a time period), an imaging target region, namely a data collection region for collecting X-ray transmission data, is not always fixed.
For example, a timing for ending data collection is usually controlled based on the number of views after the start of the collection. Hence, in a case of imaging only a certain site which is a part of an examinee (a certain imaging target region), if the timing for ending the data collection is early, the data collection region becomes narrow, so that a necessary amount of X-ray transmission data cannot be obtained. Reversely, if the timing for ending the data collection is late, the data collection region becomes wide, so that the X-ray transmission data are collected more than necessary. In this way, the data collection region for collecting the X-ray transmission data is not always fixed, which makes it difficult to accurately image an imaging target region.